You don't think the UI is part of a platform definition?
So for iOS the UI and the associated API it exposes is not part of the "platform" defintion? For osX? For win8 the UI and the associated API it exposes is not part of the "platform"?
I think the exact opposite. I think right now, to be competitive in the marketplace as a platform for a range of consumer oriented computing devices that are meant to be extendable with add-on application software.... the "platform" definition is generally expected to include the UI to build a bridge to those application developers.
I believe that "Canonical" is opting to build a distinctive platform..yes.
I believe there are compelling strategic business realities which make that sort of differentiation inevitable really. OEMs want a partner, and that partner needs to pitch a distinctive platform to be compelling to those OEMs.
I think its inaccurate to claim that "Ubuntu: The project" or "Ubuntu: The community" is attempting to build a platform. The platform building effort is very much a Canonical strategic effort. "Ubuntu: The project" and "Ubuntu: The community" to a large degree are trying to maximize what is offered under the project resource constraints. The fact that we are talking about "Ubuntu" building a platform at all is just the side-effect of the choice Canonical has made to intermix community/project and product interests under a common brand. I'm somewhat hopeful that the Unity brand becomes stronger and that they start pushing the Unity brand as the platform brand instead of overloading the Ubuntu brand. Calling it the "Unity Platform" is much cleaner strategically than calling what they are building the "Ubuntu Platform."
That being said, I think the Unity platform, would be made stronger in the marketplace if Ubuntu adopted systemd, even at this late date.